1. Field of the Invention
The present invention relates generally to a frequency detector and, more particularly, to an apparatus for detecting the frequency of an input signal with a digital scanning-converter.
2. Description of the Prior Art
Recently, computers with various functions have been developed and widely used, but horizontal synchronizing signals of video signals delivered from these computers have not yet been standardized. Therefore, a scanning-converter apparatus is required to enable a single monitor to check or monitor various input video signals having various horizontal frequencies. For such a scanning-converter apparatus, the assignee of the present application has previously proposed a scanning-converter apparatus that can be used in a monitor system of the type shown in FIG. 1. This previously-proposed scanning-converter apparatus is disclosed in Japanese patent application No. 61-250592.
FIG. 1 illustrates in diagrammatic form a monitor system that includes a television tuner 1, a high density personal computer 2 and another computer 3. The television tuner 1 produces a video signal having a horizontal frequency f.sub.H1 selected to be 15.75 kHz, the personal computer 2 produces a video signal having a horizontal frequency f.sub.H2 selected to be 24 kHz, and the computer 3 produces a video signal having a horizontal frequency f.sub.H3 selected to be 27.15 kHz. These video signals are supplied to a scanning converter apparatus 4, in which they are converted into a video signal having a horizontal frequency of 63.35 kHz, which is fed to a monitor 9 whose horizontal deflection frequency f.sub.H is 63.35 kHz.
When the incoming video signal from, for example, the computer 3 is received and reproduced by the monitor 9, seven horizontal scanning lines of the output video signal from the scanning converter apparatus 4 have to be provided corresponding to three horizontal scanning lines of the input video signal to the scanning-converter apparatus 4, because the ratio between the horizontal frequency f.sub.H3 of the incoming video signal from the computer 3 to the horizontal deflection frequency f.sub.H of the monitor 9 is EQU f.sub.H3 :f.sub.H =27.15 kHz:63.35 kHz=3:7
Accordingly, an interpolation computation must be carried out in order to obtain a sufficiently smooth and natural looking output picture image. The coefficient used in the interpolation computation depends on the horizontal frequency of the input video signal, so it is necessary to provide a circuit that can accurately detect the horizontal frequency of the input video signal. The circuit illustrated in FIG. 2 is representative of a prior-art frequency detector that has been proposed for this purpose.
As FIG. 2 shows, a horizontal synchronizing signal HD of the input video signal is supplied to an input terminal 10. The horizontal synchronizing signal HD has a frequency of f.sub.H. The input terminal 10 is connected to an input terminal IN of a monostable multivibrater 11 so that, in response to each leading edge of the horizontal synchronizing signal HD, the monostable multivibrator 11 produces at its output terminal Q a pulse train 11a of a predetermined pulse width T. This pulse train 11a is converted into a direct current signal 12a by a low-pass filter (LPF) 12 and is then fed to a level detector 13. The level detector 13 is composed of a plurality of comparators (not shown) having different comparison or reference levels, and only a comparator with a reference level higher than the level of the direct current signal 12a produces an output of high level "1". The output signal is directly supplied to output terminals 14a, 14b, . . . 14z. When the horizontal frequency f.sub.H of the horizontal synchronizing signal HD is increased and its pulse interval is reduced, the value of the direct current signal 12a, which is the output signal from the low-pass filter 12, is increased so that the frequency f.sub.H of the horizontal synchronizing signal HD can be identified stepwise by detecting which one of the output terminals 14a to 14z produces an output signal of high level "1".
In the prior art frequency detector as described above, however, a constant number of the output pulses 11a of the monostable multivibrator 11 is required, and when the frequency of the horizontal synchronizing signal HD constituting the signal to be measured is increased beyond a certain point, the pulses in the pulse train 11a from the monostable multivibrator 11 overlap one another, causing a detection error.
Particularly, when an incoming video signal is derived from a cassette-type VTR (video tape recorder) or the like, it is frequently observed that a fluctuation (jitter) of pulses exists in the time base of the horizontal synchronizing signal HD or a pulse dropout 10a occurs therein as shown in FIG. 2. This jitter or pulse dropout is directly detected as a detection level change by the prior-art frequency detector, so that the detected value becomes inaccurate and unstable. If such detected value is employed for signal processing, a deleterious influence will be exerted upon a peripheral circuit in which the detected value is employed.